From 94d99a2355e32ba8eae7bdfdf861f90286634cd9 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Thu, 11 Apr 2019 10:06:20 -0700 Subject: [PATCH] mm: make page ref count overflow check tighter and more explicit commit f958d7b528b1b40c44cfda5eabe2d82760d868c3 upstream. We have a VM_BUG_ON() to check that the page reference count doesn't underflow (or get close to overflow) by checking the sign of the count. That's all fine, but we actually want to allow people to use a "get page ref unless it's already very high" helper function, and we want that one to use the sign of the page ref (without triggering this VM_BUG_ON). Change the VM_BUG_ON to only check for small underflows (or _very_ close to overflowing), and ignore overflows which have strayed into negative territory. Acked-by: Matthew Wilcox Cc: Jann Horn Cc: stable@kernel.org Signed-off-by: Linus Torvalds Signed-off-by: Greg Kroah-Hartman --- include/linux/mm.h | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) diff --git a/include/linux/mm.h b/include/linux/mm.h index 58f2263de4de..4023819837a6 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -824,6 +824,10 @@ static inline bool is_device_public_page(const struct page *page) #endif /* CONFIG_DEVICE_PRIVATE || CONFIG_DEVICE_PUBLIC */ +/* 127: arbitrary random number, small enough to assemble well */ +#define page_ref_zero_or_close_to_overflow(page) \ + ((unsigned int) page_ref_count(page) + 127u <= 127u) + static inline void get_page(struct page *page) { page = compound_head(page); @@ -831,7 +835,7 @@ static inline void get_page(struct page *page) * Getting a normal page or the head of a compound page * requires to already have an elevated page->_refcount. */ - VM_BUG_ON_PAGE(page_ref_count(page) <= 0, page); + VM_BUG_ON_PAGE(page_ref_zero_or_close_to_overflow(page), page); page_ref_inc(page); }